1. Field of the Art
The present invention relates to a method and an apparatus for cooling window foils in an electron beam accelerator, and more particularly, to a method and an apparatus for cooling window foils in a scanning type electron beam accelerator containing double windows used to treat a boiler exhaust gas and the like.
2. Prior Arts
Electron beam accelerators are widely used in various industrial fields. Among them, in electron beam accelerators used for treating a subject to be irradiated in the atmosphere such as a boiler exhaust gas, electron beams accelerated to a high speed in vacuum of the accelerator must be extracted into the atmosphere. For this purpose, a window formed of a pure titanium or titanium alloy foil 30 to 50 microns thick is utilized. Further, when the density of an electron current is increased to enhance the irradiation effect, an accelerator with an electron current of several hundreds of milliamperes must be used.
In this kind of accelerator, part of the energy is absorbed when electron beams pass through a window foil whereby heating results and the foil may be damaged. To prevent this, a window for extracting the electron beam is formed in a rectangular shape and the electron beam is scanned along a long axis of the rectangular shaped window to prevent the electron beam from being concentrated in one position on the foil. At the same time, a nitrogen gas or the like is blown against the window foil to cool it and prevent unwarranted temperature rise.
Further, when the electron beam has a current value as large as several hundred milliamperes, the electron beam is further scanned along the short axis of the rectangular shaped window to increase the travelling distance thereof thereby increasing the cross sectional area of the rectangular shaped window. However, when the cross sectional area of the window portion is increased, since the foil is used to isolate a vacuum in which the electron beam is accelerated from the atmosphere, a force acting on the foil increases, thereby causing the foil to be pulled into the vacuum side excessively. To prevent this, a support is provided at the center of the window foil to hold the foil. Consequently, the window portion is divided into two regions. Such a structure is known as a double window.
Further, double foils including a primary foil and a secondary foil are used in the window to protect the window foil in processes such as treating a boiler exhaust gas in which sulfuric acid and nitric acid could be produced.
FIG. 3 shows the arrangement of the conventional electron beam accelerator, FIG. 4 shows the structure of the double windows and FIG. 5 shows a conventional window foil cooling mechanism (a view in a short axis direction of the window). In the figures, the reference numeral 1 denotes a high voltage power supply, 2 a high voltage cable, 3 an accelerating tube, 4 a scanning coil, 5 a scanning tube, 6 a window foil mounting flange, 7 an exhaust gas duct, 8 an electron beam, 9 an exhaust gas flow, 10 a first window of primary window foil, 11 a second window of primary window foil and 12 a locus of scanned electron beam. In the conventional electron beam accelerator using the double windows or the double window foils, the primary foil 14, 15 and the secondary foil 16 are cooled by blowing gasses from respective sides of the rectangular window along the long axis direction thereof confronting with each other. In this case, since both the double windows of the primary foil must be simultaneously cooled, a central support 13 is retracted from the end surface of a window mounting flange 25 to the vacuum side so as to cause cooling gasses 19, 20 to impinge on both the first window 14 and the second window 15 of the primary window foil.
The cooling gasses 19, 20 are supplied from corresponding blowing out slits 17, 18 for cooling the primary window and the secondary window, respectively.
When the distance of the window foil is increased in the short axis direction or width thereof to increase the scanning distance of the electron beam in the conventional cooling method, the second window 15 of the primary window foil may be excessively heated and damaged because the cooling gasses do not sufficiently reach the second window of the primary window foil. An increase in the amount of the cooling gasses to prevent this problem requires a blower having a large capacity and is economically unrealistic.
Further, in a conventional both-side-blowing-out cooling method wherein a gas reversing mechanism 29 is mounted to the central support 13 disposed in a vacuum side from the atmospheric side as shown in FIG. 6, since a window foil is sandwiched between the central support 13 and reversing mechanism 29, the reversing mechanism cannot be directly mounted on the central support at the portion of the window foil. Thus, since the reversing mechanism 29 is long (e.g. 1-3M) and is separately mounted on the central support at the short axis portion of the window, a reversing mechanism 29 is heated and expanded resulting in deformation by a dispersed electron beam impinging thereon while an accelerator is in operation, so that a gap is formed between the central support and the reversing mechanism. As a result, the gap may allow gas to leak, and cooling gasses may not be able to be reversed smoothly, the gap may clog with dust, the flow is disturbed and the foil may be damaged in some cases. In addition, the cooling mechanism shown in FIG. 6 is not used in the application in which a secondary window foil is used.
Therefore, an object of the present invention is to provide a method and apparatus for cooling window foils which are capable of sufficiently cooling window foils even if the length of a window portion is increased in the short axis direction thereof to increase the travelling distance of electron beams.
Another object of the invention is to provide a method and apparatus for cooling window foils as described above and which is further capable of avoiding deformation of the reversing mechanism caused by impingement of the dispersed electron beam and thereby maintains smooth reversal of the flow of the cooling gasses.
Further object of the invention is to provide a method and apparatus for cooling window foils as described above and which is further capable of cooling both the primary window foil and the secondary window foil simultaneously without the need for large equipment.